It is generally known that liquids, particularly aqueous solutions, such as water for various industrial uses, must be practically free of salts or its ionic dissociation components.
To serve this purpose, various processes have been used for the treatment of such liquids whereby such dissolved salts are removed.
Treatments having long been known for the removal of salts dissolved in water by absorbing the ions onto suitable substances, e.g., ion-exchange masses. However, this method has the considerable disadvantage in that the exchange masses get depleted over and over again and must be regenerated with dangerous chemicals such as acid or caustic solutions. Furthermore, considerably higher amounts of these chemicals must be used when compared with the amount of eliminated salt. Such dangerous chemicals are then flushed out with the waste water and constitute an environmental strain no longer acceptable to today's society, e.g., destruction of the biosphere in surface waters and the like.
It has also been known that ion-exchange masses can be regenerated with the aid of electric energy. Up until now, the practical use of this method was hindered by the fact that the energy requirement for producing sufficient capacity to separate the salt ions is relatively high, and that the apparatuses employed are much too expensive and do not always conform to the technical and industrial requirements.
Also known is a process called "electrodialysis". In this process, the ion-permeating or ion-blocking effects of ion-exchange membranes are used in such way that the ions of the liquid (or aqueous solution) to be demineralized migrate apart in an electric field through the membranes. This results in areas of salt enrichment and salt reduction by appropriately arranging the membranes. Considerable drawbacks have been noted both in the process and the apparatus employed for electrodialysis. As the water is demineralized its salt content decreases during the process. However, the electrical resistance rises in such way that an economical operation attaining high degrees of demineralization cannot be achieved with such known process. Thus, in order to attain an application at least in the area of low product grades, the membranes have been arranged at extremely small distances of not much more than 1 mm from each other. Through this close arrangement of the membranes the electric resistance, and with it also the consumption of energy, can be diminished; however, in practice, equipment costs for supplying and removing the liquid streams of product and brine rise considerably, and the susceptibility to fouling within the narrow passages between the membranes comprises the safe operation of the device.